Light stimulus change evokes an activity response in the rat

Bright light may be a danger signal for rats because they are more vulnerable to predators in bright environments. We examined the fear-evoking properties of bright light with a novel open-field procedure that confronts a rat with the sudden onset or termination of a bright light gradient. The rats did not freeze but exhibited a transient increase in locomotion to light onset and termination, which we call the activity response (AR). This finding suggests that the AR is an exploratory response geared at investigating stimulus change. The rats also displayed a preference for dark to the lighting differential that was not due to the novelty or slight heating differential supplied by the lamps. These experiments demonstrate that the sudden onset of bright light engages preencounter defensive behavior, as described by the predatory imminence model (Fanselow & Lester, 1988). This task is amenable to studying light-evoked defensive responses.